Also, the email address to which the message was sent appears clearly in the "To:" header.
Either you or your friend is misspelling their address (more common than you might think, I get opt-in mailing-list mail for email@example.com, intended for firstname.lastname@example.org), or you've encountered a bug.
The dots are for you to play around with, but the mail all goes to one account. Or, at least, that's how it works with my account.
Google's own email@example.com mailing list has this as the "To:" field:
And at the bottom of the message:
To unsubscribe from this group, send an empty message.
I had to ctrl-u and check the "Delivered-To:" and "X-Forwarded-For:" headers before I could unsubscribe.
(I'd tried to unsubscribe previously but the subscribed email account forwards to my main account so replying with an empty message didn't work. This thread prompted me to dig a little deeper and finally get one less piece of email per day - thanks HN!)